At the present time, electrical power is available to virtually all people in most civilized countries of the world. Consumers of such electrical power are often distributed over a wide geographic area while power generation facilities are generally located in the proximity of either a fuel or energy source (e.g. hydroelectric facilities are often located near where a water reservoir naturally exists or can be economically constructed) or population centers such as cities although nuclear reactors used for power generation are often located somewhat more remotely.
All such power generation facilities inherently have limited power generation capacity although that capacity may be quite large. Many facilities are most efficient when operating continuously near their full power generation capacity while demand for electrical power can be quite variable. At the same time, greater power generation capacity of a given facility may greatly increase the initial capital expenditure required as well as possibly increasing cost of maintenance over the service life of power generation equipment. Therefore, while it is desirable to provide electrical power to consumers located near a power generation facility to limit inefficiency due to power transmission losses and to limit capital expenditures by limiting the power generation capacity of respective facilities to a small excess capacity over anticipated peak demand, it is also desirable to interconnect many such power generation facilities so that excess generated power at a given location can be distributed to locations where demand may, from time-to-time, exceed local power generation capacity. Such interconnection infrastructure is generally referred to as a power grid or, simply, grid and requires that power generation facilities be carefully and precisely synchronized in both frequency and phase so that power can be exchanged between the grid and the local power generation and distribution network.
It is also critical that a connection between local power generation equipment or facility and the grid be maintained, not only to allow frequency and phase information of grid power to be obtained but to avoid excess power being delivered to the grid from being redirected to local loads by a disconnection. Such a disconnection, sometimes referred to as grid loss, can rapidly cause significant damage to local loads and local power converters must be rapidly shut down when a disconnection is detected to prevent or mitigate such damage.
A lack or loss of grid connections is referred to as islanding and the likelihood of disconnection has been aggravated in recent years by the proliferation of relatively small power generation facilities deriving energy from so-called renewable resources such as solar and wind power that may not be consistently available. Such systems usually generate and/or store power as a direct current (DC) voltage and use a controllable converter to derive alternating current (AC) for transmission. Therefore stringent standards have been promulgated for detection of loss of synchronization and disconnection of a local network from the grid.
At the present time, the standard for detection of islanding and providing anti-islanding protection is the IEEE 1547 standard which requires that any distributed power generation facility under 10 MW capacity must be able to detect islanding and de-energize the area electric power system (EPS) within two seconds. The test load specified by the standard is a paralleled RLC (//RLC) load which is resonant at 60 Hz (or the frequency that may be used for the grid) which represents a worst case for islanding detection since such a load presents a near-zero impedance, similar to the impedance of the grid, at the resonant frequency. (An ideal grid would exhibit zero impedance and a grid exhibiting any significant impedance is referred to as a weak grid. The limiting case of grid weakness would be a grid exhibiting infinite impedance and would appear substantially identical to a disconnection from the grid although some voltage or phase information might still be derived.) The standard also requires so-called low-voltage ride-through (LVRT) to accommodate a condition when the grid voltage drops but the grid connection is maintained such that the local power generation facility can and should continue to deliver power to the grid. Islanding detection should also achieve an almost zero non-detection zone (NDZ) such that virtually no islanding condition or event can exist or occur without detection.
Islanding detection arrangements can be either active or passive. Passive islanding detection schemes passively search for disturbances on the grid through sensed and calculated system parameters. The main drawback of passive islanding detection schemes is exhibiting different NDZ regions. In particular, no passive islanding detection arrangement can detect islanding when the load matches the output active power of the converter at the same resonant frequency as the line frequency of the grid.
Among active islanding detection arrangements, output-frequency based islanding detection (OFID) methods that detect changes in frequency and/or phase between the grid and local power generation equipment have been of substantial interest since, in general, they do not violate the LVRT requirement and can provide an almost zero NDZ. Many OFID methods are known that make modifications to the voltage or current control loop of converters and thus are configured to generate so-called frequency positive feedback that will drive the converter system frequency away from the steady state frequency when a reference frequency signal from the grid is not available. However, suitable positive feedback mechanisms and characteristics and design procedures for such methods are not well-developed at the present time and over-design or excessive experimentation have often been required to meet the islanding detection standard. While approaches to islanding detection have recently been the subject of substantial study, few studies have considered the impact of OFIDs on power converter operation, entire system stability or performance of sophisticated power systems such as multi-converter systems. None of the known OFID approaches have, to date, achieved a robust zero NDZ.